The present invention relates to a charge amplifier circuit with an operational amplifier, an integrating capacitor between the inverting input and the output of the operational amplifier, and a resetting device for discharging the integrating capacitor.
Charge amplifiers as used, for example, in conjunction with piezoelectric transducers suffer from the disadvantage that their output voltage is affected by a zero shift or drift. Under certain circumstances, this fault may influence measurements adversely. It is known that the magnitude of this error and its effects on the measuring accuracy depend primarily on the quality of the input circuit and of the input stage of the charge amplifier. Among other things, the input stage must have an extremely high input resistance (typically 10.sup.14 ohms). The zero shift of the output voltage is relatively small and stable with a charge amplifier having a stable input stage, typically assembled from junction field-effect transistors (JFETs). However such input stages are subject as a rule to leakage currents which cannot be ignored, which moreover are greatly dependent on temperature, and which cause drift in the output voltage. On the other hand, input stages assembled from MOSFETs, for example, have small leakage currents, so that charge amplifiers equipped with them show less drift in the output voltage and are thus suitable also for quasi-static measurements, whereby drift compensation is not necessary in most cases. Nevertheless, MOSFET input stages have relatively poor stability. Owing to this circumstance, the zero shift of the output voltage varies with time and temperature, necessitating frequent correction if measurement is to be exact.
The object of the invention is to provide a charge amplifier circuit comprising the means named above, which effects automatic compensation of the zero shift in the output voltage and can be used on a charge amplifier with an input circuit having an extremely high resistance.
Direct-current amplifiers are known in which correction of the zero shift and other errors can be accomplished by feeding a signal back one or more times to the amplifier input after a prior feed of the signal to be amplified (DE-PS Nos. 1 562 070 and 24 32 404). In order to achieve the successive working phases of one or more amplifier stages, at least one store circuit, at least one feedback loop and several switches operating periodically must be provided. Apart from the relatively large outlay in circuitry, such circuit arrangements are unsuitable for use in a charge amplifier circuit with an extremely high ohmic input stage, and are therefore unsuited for the task involved here. In particular, the measuring operation would be disturbed by switching operations in the input circuit of the charge amplifier, and by additional loads on the amplifier input during the measuring phase caused by feedback loops.
By contrast, according to the invention, the task in question is performed by having at the output from the charge amplifier circuit a circuit arrangement with a correction amplifier, which when the resetting device is activated, automatically feeds a correcting charge to the integrating capacitor that compensates the zero shift in the output voltage of the charge amplifier circuit during the measuring phase following the reset phase.
The invention thus shows a way to accomplish correction of the zero shift on a charge amplifier circuit having an extremely high ohmic input stage resistance, by relatively simple means involving small expenditures. The integrating capacitor provided renders separate store circuit superfluous. Interruption of the input signal and additional loading of the amplifier input are avoided.
From AT-PS No. 377 132 a charge amplifier circuit is known, in which a correcting variable derived from the output voltage is likewise fed back to the charge amplifier input during the reset phase, becoming effective in the measuring phase that follows. However, this is a drift compensation, which compensates the leakage currents in the input circuit of the charge amplifier with a correcting current. For this an amplifier, a controlled A/D converter and a D/A converter are needed in the feedback circuit. In order to retain the correcting current during the measuring phase, the D/A converter must be connected permanently with the charge amplifier input. A circuit arrangement of this kind can therefore lead to measuring errors owing to the additional loading of the input circuit during the measuring phase, especially under dynamic operations with a charge amplifier having an extremely high ohmic input stage. Moreover, in view of the relatively high outlay entailed by the drift compensation according to AT-PS No. 377 132, compensation of the zero shift in accordance with the present invention constitutes a simpler and less expensive way of correcting errors where there is a choice available between using an unstable charge amplifier input with extremely high resistance and a stable one with less resistance.
Another prior art is to give an initial charge to the integrating capacitor before the measuring phase with an operational amplifier acting as integrator (U. Tietze & Ch. Schenk, Halbleiter-Schaltungstechnik, 3rd Edition, 1974, pages 241 and 242). For this purpose, with the signal source switched off, the integrating capacitor is charged by a separate voltage source. However, no feedback of the output voltage from the amplifier is provided to set up an initial charge for correction purposes. Moreover, for reasons already stated, cutting-out the signal source during the reset phase is undesirable, nor is it necessary in the case of the present invention.
A practical embodiment of the charge amplifier circuit according to this invention has the output of the charge amplifier circuit connected with the input of the correction amplifier, while the integrating capacitor is by-passed by a resistor when the resetting device is activated, and the output of the correction amplifier is connected with the inverting input of the operational amplifier via a further resistor. The resetting device preferably includes a highly insulating switch (e.g., a Reed relay), which when closed connects the two resistors with the inverting input of the operational amplifier.
The accuracy attainable by the automatic zero shift correction depends on the zero data of the correction amplifier. It is therefore of advantage to employ for this an operational amplifier whose output voltage zero drift is smaller and more stable than that of the operational amplifier forming the charge amplifier. On the other hand, the frequency response of the correction amplifier is not critical, because the reset phase lasts relatively long and the zero shifts of the charge amplifier generally proceed very slowly. The control loop which includes the correction amplifier is preferably stabilized in addition by an integrating element.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawing which shows, for purposes of illustration only, one embodiment of the present invention.